A systematic review of structure-activity relationships in bacterial levan: physicochemical mediation and context-dependent functionality.

Bacterial levan is a β-(2 → 6)-linked fructan with variable β-(2 → 1) branching, molar-mass distribution and higher-order organization. Although these descriptors are frequently associated with antioxidant, anti-inflammatory and prebiotic outcomes, the evidential basis for direct structure-activity relationships remains uncertain. We systematically reviewed 26 primary studies identified through Scopus and Web of Science Core Collection using a PRISMA 2020-informed process. Eligibility was separated from post-inclusion appraisal, and studies were classified according to whether they provided direct within-study evidence, indirect triangulated evidence or contextual observations only. Evidence was synthesized separately for chemical antioxidant assays, cell-based inflammatory models, microbial fermentation/prebiotic systems, gastrointestinal-protection models and material-performance tests. Across regimes, molar mass alone did not consistently explain activity, and few studies jointly measured polymer structure, physicochemical state and functional outcome. Diffusion, aggregation, molecular accessibility and interfacial interactions are therefore not treated as established mediators; rather, they form a testable interpretive model consistent with the available indirect evidence. Low-resolution studies were retained to characterize the evidentiary landscape but were excluded from positive mechanistic inference. This review defines the minimum analytical and experimental conditions required to test levan structure-function hypotheses and provides a chemistry-centred framework for future, within-system validation.